Patent GB-959555-A (GB)
Title: Improvements in or relating to data transmission systems
Inventor: WRIGHT ESMOND PHILIP GOODWIN
IPC: H04M11/06
Filed: 1960-01-21
Abstract: 959,555. Telegraphy; exchange systems; line transmission systems. STANDARD TELEPHONES & CABLES Ltd. Jan. 20, 1961 [Jan. 21, 1960], No. 2178/60. Headings H3K, H3P and H3R. In a system for the transmission of information between two stations, the channel bandwidth is divided into unequal bandwidth subchannels, the sub-channel of greater bandwidth being allocated to the transmission direction which carries the greater proportion of the total traffic over the channel. The wide-band sub-channel is used for the transmission of blocks of data, and the other subchannel is used for the return transmission at low speed of redundancy signals which are compared with redundancy signals at the transmitter to determine whether or not the data block has been received correctly and, in the event of incorrect reception, to initiate retransmission. Fig. 1 illustrates the complete system, comprising stations A and B. Station A calls station B by dialling. When the call is answered at 10b subscriber A orally informs subscriber B that he has data for him. Switches 13a, 13b are then reversed and a start signal is sent via modulator 25, low-pass filter 21b, and hybrid 17b to line. At station A the signal passes via hybrid 17a, filter 21a and demodulator 22 to light lamp 30a, to indicate that station B is ready to receive. The low-pass filters correspond to the narrow band low-speed path. Subscriber A now closes his start key 20, and a preamble data block, comprising signals indicative of the type of transmission and a synchronizing signal, passes via modulator 15, high-pass filter 16a and hybrid 17a to line. At B the preamble is passed via filter 16b to demodulator 24. The high-pass filters correspond to the wide-band high-speed sub-channel. Unit 27 evaluates the parity of the words in the data block, the resulting parity word being returned to A over the low-speed channel. The parity word is compared in unit 23 with the parity word stored there, derived from the transmitted block. If the comparison indicates correct reception data store 14 is now connected to modulator 15 and data blocks are transmitted to B over the high-speed channel. A supervisory signal follows each block and indicates whether or not the previous block was received correctly and can be read into the receiver main store. The bits of the words in each block are interleaved. The final data block may not be completely filled with message signals, so additional " dummy " data si added to make up the block. This block. is followed by a block which indicates where the message signals ended in the previous block. In an arrangement of station B, Fig. 2 (not shown), which permits unattended operation, the calling ringing tone from A causes a record unit to pass an oral message to A. This is followed by a tone causing a lamp to be lit at A whereupon the operator there switches over to automatic transmission of data. Station B has automatically switched to data transmission under control of a timing arrangement associated with the record unit, and so data can be exchanged as detailed above.